This invention relates generally to a method and apparatus for decoding a received digital signal using a maximum likelihood sequence estimation (MLSE), or trellis, decoder under common fading channel conditions.
The Viterbi algorithm is a well-known type of MLSE decoding method that can be used to estimate transmitted digital sequences from a signal received over a communication channel. Initial data (training symbols) decoded by a Viterbi decoder are used to construct an initial channel estimate that is then used to decode the body of the received signal. As the received signal is used to construct a trellis, the state transition metric of each state transition from a time t=(nxe2x88x921)T to a consecutive time t=nT in the trellis is either discarded as being invalid according to criteria, such as the minimum mean-squared error criteria, or used to modify the channel estimate for future state transition computations. At the conclusion of the received signal, the trellis is back-traced to obtain the estimated transmitted digital sequence.
If the channel estimate is not accurate initially or during a later portion of the decoding process, due to changing conditions of the communication channel or other reasons, the decoding of the body of the received digital signal can get progressively erroneous. An error in the initial channel estimate may cause the Viterbi decoder to modify the channel estimator in a direction that does not properly track the dynamic communication channel conditions. Additionally, any later-occurring error in the dynamic channel estimate may cause unrecoverable divergence from a replication of the actual dynamic communication channel conditions.
A variation of the above-described conventional Viterbi decoder uses a separate and independent channel estimate for each state in a trellis decoder rather than a single channel estimate for the entire Viterbi decoder. As the trellis is traversed during the Viterbi decoding process, channel estimates for each state are computed from a time t=(nxe2x88x921)T to a time t=nT. When the received digital signal ends, the best cumulative channel estimate is used to determine the estimated transmitted digital sequence by back-tracing through the trellis. Thus, an initial channel estimate can be modified in multiple directions, which reduces the chances that an inaccurate initial channel estimate will result in future channel estimates that progressively worsen. U.S. Pat. No. 5,432,821 issued Jul. 11, 1995 to Polydoros et al. proposes such a full-parallel Viterbi approach and contrasts it with a conventional Viterbi decoder.
Because full-parallel Viterbi processing creates an independent estimate of the communication channel for each state, and each channel estimate requires updating and tracking during each state transition, full-parallel Viterbi processing greatly increases the computational power required to decode a received signal. Thus, there is a need for improved MLSE decoding of received signals compared to conventional Viterbi decoding but with reduced computational complexity compared to full-parallel Viterbi processing.